Model #WFMC3200UC/01, this full sized front load washer would intermittently not lock the door and make a buzzing sound. If the customer slammed the door, they found it worked better then just closing the door. The unit worked this way for a while, then the noise stopped and the door wouldn't lock at all.
Front load washers use some method of locking the door during the wash cycle to prevent accidental opening. These locks can be actuated by a couple electrical components called solenoids and wax motors. Whichever method is used, the purpose is to lock the door and have it remain locked until the washer has completed the entire cycle.
According to the customer, whenever the door didn't lock, there was a buzzing noise from the area of the latch which was most likely caused by the solenoid not being able to fully actuate. Because the noise has since gone away and the door won't latch, my guess was the solenoid eventually overheated and failed. Solenoids use a wire coil to generate a magnetic field which moves an iron shaft either in or out. If the shaft is prevented from moving, the heat generated will eventually damage the coil and the solenoid will stop working.
On this unit, I removed the outer door bellow (door seal) mounting spring so I could get to the latch assembly on the right side. A couple screws and the latch was in my hand still connected to three wiring harnesses. Using my meter, I did a resistance check of the solenoid and found an open circuit where resistance from the solenoid should be. No current flow, no magnetic field. I replaced it with a new lock assembly and now the door locks properly again.
Just a note: If your Bosch washer buzzes when the door tries to latch, press in on the door near the latch to ensure it is closed and locks properly. Often times, the doors will sag enough to cause the door strike to actuate the switch, but not be fully seated into the lock mechanism. Allowing the buzzing to continue will eventually result in a solenoid failure.